Bump the immutables dep to the latest

Yury Selivanov · Yury Selivanov · commit 1772052db944 · 2019-04-01T10:40:41.000-04:00
diff --git a/setup.py b/setup.py
@@ -17,7 +17,7 @@
     packages=['contextvars'],
     provides=['contextvars'],
     install_requires=[
-        'immutables>=0.6',
+        'immutables>=0.9',
     ],
     license='Apache License, Version 2.0',
     classifiers=[
